The present invention relates to liquid crystal display elements and more particularly to a method of making liquid crystal display elements having a precisely controlled, uniform cell gap.
Use of liquid crystal display (LCD) elements has begun to emerge as the method of choice for displaying large amounts of graphical information in displays such as computers and video displays in addition to the use of LCD's in their traditional role as a display unit for devices requiring limited display of information such as in watches, cellular telephones and the like. Traditionally, liquid crystal display elements have been manufactured by printing an array of electrodes on a first substrate, printing a corresponding transparent electrode or electrodes on a second, glass substrate, and injecting a liquid crystal material between the two substrates. The separation between the two substrates, which essentially determines the thickness of the liquid crystal layer, should be held as constant as possible over the entire area of the cell. Traditionally, the cell gap is maintained through the introduction of spacers between the substrates. Typically, the spacers comprise tiny particles such as glass or plastic beads, glass fibers, or carbon fibers. The particles may be dusted onto the substrate by exposing the substrate to an atmosphere containing a particular concentration of the particles, or may be spun onto the substrate by depositing on the substrate a solvent carrying a particular concentration of the particles and centrifuging the substrate to distribute the particles and evaporate the solvent. With either of these methods of application, a combination of electrostatic and steric forces, primarily electrostatic forces, cause the particles to adhere to the substrate.
For optimum performance, the cell gap should be maintained at the optimum distance with no tolerance. Unfortunately, it is not commercially practicable to manufacture liquid crystal display elements with zero tolerance on the cell gap. Commercially available spacers having a particular nominal size will, of course, in reality constitute a distribution of particles having a mean particle size and particles that are larger and smaller than the mean particle size. Since, as the substrates are brought together, the largest particles in the distribution will contact the substrates first, it is the size of the largest particles that primarily determines the cell gap. Moreover, since the substrates are usually flexible, at least where the cell gap is on the scale of the 0.4 to 10 microns and the ratio of cell span to cell gap is very large, as is required for high performance microdisplays, the distribution of particle sizes within the display element will allow the cell gap to vary across the display element allowing an inhomogeneous thickness of the liquid crystal layer. The inhomogeneous thickness results in optical path length differences (the product of the birefringence of the liquid crystal and the cell gap) across the display, resulting in a deleterious effect on the contrast ratio and the chromatic fidelity of the display.
New and improved liquid crystal materials and high performance substrates are being developed for high-speed, low operating voltage displays. Substrate spacings of less then five microns will be required for these new and improved displays. As the mean value of the cell gap is further and further reduced, the sensitivity of these devices to variation in cell gap becomes more and more critical. Various methods have been suggested for improving cell gap uniformity. U.S. Pat. No. 5,210,629 discloses a method of filtering the glass spacers to improve spacer uniformity. U.S. Pat. No. 4,653,864 discloses a method of forming polyimide spacers on a substrate using conventional photolithographic techniques. U.S. Pat. No. 4,626,073 discloses use of elastic spacers in lieu of conventional rigid glass or polymer spacers. What is needed, however, is a method of improving cell gap uniformity without the added expense of additional photolithographic process steps or cumbersome filtration techniques.